Abstract

We have performed detailed thermophysical and dynamical modelling of the
Jovian Trojan (1173) Anchises. Our results show that this is the most
unusual object. By examining observational data of Anchises taken by
IRAS, Akari and WISE at wavelengths between 11.5 and 60 mum, together
with the variations in its optical light curve, we find that Anchises is
most likely an elongated body, with an axis ratio, a/b, of around 1.4.
This results in calculated best-fitting dimensions for Anchises of 170
× 121 × 121 km (or an equivalent diameter of 136 +18/-11
km). We find that the observations of Anchises are best fitted by the
object having a retrograde sense of rotation, and an unusually high
thermal inertia in the range 25-100 J m-2 s-0.5
K-1 (3sigma confidence level). The geometric albedo of
Anchises is found to be 0.027 (+0.006/-0.007). Anchises therefore has
one of the highest published thermal inertias of any object larger than
100 km in diameter, at such large heliocentric distances, as well as
being one of the lowest albedo objects ever observed. More observations
(visual and thermal) are needed to see whether there is a link between
the very shallow phase curve, with almost no opposition effect, and the
derived thermal properties for this large Trojan asteroid. Our dynamical
investigation of Anchises' orbit has revealed it to be dynamically
unstable on time-scales of hundreds of millions of years, similar to the
unstable Neptunian Trojans 2001 QR322 and 2008
LC18. Unlike those objects, however, we find that the
dynamical stability of Anchises is not a function of its initial orbital
elements, the result of the exceptional precision with which its orbit
is known. Our results are the first to show that a Jovian Trojan is
dynamically unstable, and add further weight to the idea that the
planetary Trojans likely represent a significant ongoing contribution to
the dynamically unstable Centaur population, the parents of the
short-period comets. The observed instability (fully half of all clones
of Anchises escape the Solar system within 350 Myr) does not rule out a
primordial origin for Anchises, but, when taken in concert with the
result of our thermophysical analysis, suggest that it would be a
fascinating target for a future study.